Variable radio-frequency coupler

ABSTRACT

THE INVENTION IS A VARIABLE RADIO-FREQUENCY, QUARTERWAVE, TEM MODE COUPLER WITH PARALLEL GROUND PLANES AND CENTER CONDUCTORS OF ANY CONVENTIONAL SHAPE. VARIABLE COUPLING IS ACCOMPLISHED BY VARYING THE GROUND-PLANE SPACING IN THE COUPLING SECTION.

Filed Nov. 18, 1968 FIG- 2 e W w w United States Patent O 3,560,885 VARIABLE RADIO-FREQUENCY COUPLER Gene Chao, Menlo Park, Calif., assignor t Textron Inc. Belmont, Calif., a corporation of Rhode Island Filed Nov. 18, 1968, Ser. No. 776,372 Int. Cl. H01p /14 US. Cl. 333 6 Claims ABSTRACT OF THE DISCLOSURE The invention is a variable radio-frequency, quarterwave, TEM mode coupler with parallel ground planes and center conductors of any conventional shape. Variable coupling is accomplished by varying the ground-plane spacing in the coupling section.

BACKGROUND OF INVENTION It is conventional to couple radio-frequency energy between coaxial or stripline circuits, by couplers employing the transverse electric and magnetic field mode (TEM mode). A basic coupling section is one-quarter wavelength long at center operating frequency. The bandwidth of such couplers may be extended by cascading coupling sections, either symmetrically or asymmetrically as is shown in the art. Basic couplers of this general type are commonly termed quadrature couplers, inasmuch as the two outputs differ in phase by 90. In this type of coupler, the coupled output is in phase with the input and the connected output lags the input by 90 while, theoretically, no energy appears at the fourth terminal, or port.

Physically, TEM mode couplers may be constructed with a pair of common parallel ground planes with center conductors of separate circuits disposed in close proximity to each for one-quarter wavelength of the center frequency and located midway between these ground planes. Numerous variations in physical structure are known and, for example, the central conductors may have a wide variety of configurations, such as, for example, round rods, rectangular bars, etc. It will also be appreciated that couplers of this type are equally applicable to stripline circuits for either parallel or broadside coupling, and such circuits are herein considered as a variation of coaxial lines for the purpose of the present invention.

In order to controllably vary the coupling in devices of the type briefly described above, it is conventional to provide some means of varying the spacing between center conductors of the two circuits to be coupled. Although this may be accomplished in a variety of ways, it usually requires some type of sliding contacts on at least one of the coupler arms. Difiiculties are often encountered with this type of variable coupler, inasmuch as sliding contacts are erratic and unreliable at microwave frequencies.

The present invention provides for varying the coupling in radio-frequency, quarter-wave, TEM mode couplers by varying the ground-plane spacing thereof in a conrtolled manner.

SUMMARY OF INVENTION The present invention, in brief, provides a different and highly advantageous manner of varying the coupling between R-F circuits. This is herein accomplished by controllably varying the ground-plane spacing in quarterwave TEM mode couplers. Coupling between the circuits is a function not only of the spacing between center conductors of separate circuits but also is a function of separation of such conductors from the ground planes defining the physical boundaries of the circuit. Inasmuch as the ground planes are the exterior portions of the circuits, or coupling devices, they are readily accessible for controlled movement in a very precise manner. This then 3,560,885 Patented Feb. 2, 1971 obviates prior-art difficulties of attempting to vary the spacing between central conductors in this type of coupler.

A variety of different manners of varying ground-place spacing is available. Simple embodiments of the invention include the provision of large fiat-threaded screws as ground planes on opposite sides of the coupling region, as well as other types of movable walls forming ground planes in the coupling region. In order to preclude the slight mismatch introduced by displaced ground planes in accordance with the present invention, it is furthermore provided herein that oppositely moving sidewalls may be incorporated in the coupler, and certain embodiments thereof are illustrated and described below.

DESCRIPTION OF FIGURES The invention is illustrated as to particular preferred embodiments thereof in the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a prior-art TEM mode coupler;

FIG. 2 is a schematic sectional view of a variable coupler in accordance with the present invention;

FIG. 3 is a schematic illustration in plan view of the coupler of FIG. 2 and taken, for example, in a plane 3-3 of FIG. 2;

FIG. 4 is a schematic illustration of mechanical means for moving the ground planes of a coupler toward and away from each other for controlling the coefficient of coupling in accordance with the present invention;

FIG. 5 is a schematic illustration of a modified form of the present invention, wherein all four walls of a rectangular coupler are simultaneously moved for varying the coeflicient of coupling and compensating for movement of the ground planes insofar as impedance is concerned; and

FIGS. 6 and 7 are schematic illustrations of an alternative embodiment of the present invention illustrating the ground planes in normal spaced relationship and in closer spaced relationship, respectively.

DESCRIPTION OF EMBODIMENTS Reference is made to FIG. 1 of the drawing schematically illustrating the structure of prior-art R-F couplers employing TEM mode. As illustrated, a pair of central conductors 11 and 12 are disposed closely adjacent each other for substantial coupling action, and are located between metal plates 13 and 14 normally forming the ground planes of the circuit. In order to provide for a variation in the degree of coupling between the circuits represented by the conductors 11 and 12, it is conventional to provide some means for moving these conductors relative to each other so as to vary the spacing therebetween. Commonly, this is accomplished by means of sliding contacts on at least one of the central conductors. An increased separation of the conductors 11 and 12 decreases the coupling therebetween; however, it is readily apparent that access to the inner conductors 11 and 12 is somewhat difiicult, and precise control over the spacing thereof poses problems. It has, in fact, been found that this type of variable coupler is at best erratic and often unreliable at microwave frequencies. Relative movement of the center conductors with respect to each other is generally indicated by the arrow above the conductors in FIG. 1; and suitable mounting means for the central conductor is, of course, provided as, for example, in the form of a solid dielectric between the gorund planes 13 and 14.

The present invention provides for varying the coupling in a radio-frequency, quarter-Wave, TEM mode coupler by varying the ground-plane spacing rather than the center-conductor spacing. In this respect, reference is made to FIG. 2 illustrating, schematically, portions of a coupler including central conductors 16 and 17, which are, preferably, fixed in relation to each other. For parallel coupling, the conductors 16 and 17 may take the form, somewhat, as illustrated in FIG. 3, with coupling sections being adjacent and parallel for one-quarter wavelength at center frequency. The ground planes for the coupler are formed by the top and bottom metal walls 18 and 19, as illustrated; however, in the coupling region these walls are made movable toward and away from each other. This may be accomplished in the simple manner illustrated in FIG. 2 by the provision of large flat screws 21 and 22 threaded in openings in the top and bottom walls 18 and 19, respectively. It will be appreciated that the center conductors 16 and 1-7 are to be located equidistant from the ground planes, in accordance with conventional theory; thus, in order to change coupling by varying ground-plane separation, it is necessary to move both of the ground planes toward or away from the central conductors by equal amounts. The simple schematic embodiment of the present invention illustrated in FIGS. 2 and 3 does not incorporate any means for insuring this equal and opposite movement; however, same may be provided, as described below.

With fine threads provided on the screws 21 and 22, it is possible to precisely control, or vary, the ground plane separation, and thus to precisely vary the coupling between circuits. It will, of course, be appreciated that the two circuits employ common ground planes, or exterior conductors, which are normally grounded. It is further noted that no attempt is made in the illustrations of the present invention to depict all physical details of a coupler in accordance with the present invention. Insofar as general structure is concerned, conventional coupling structure is satisfactory: this also applies to the dielectric disposedv between the conductors of the present invention and method and means for mounting the central conductors. Of course it is to be appreciated that the space between the movable ground planes, such as the fiat screws 22 and 23 of FIG. 2, is not to be completely filled with a solid incompressible dielectric, for, of course, this would prevent movement of the screws toward each other to reduce ground-plane separation. Many suitable dielectrics for use with coaxial cables and the like are well known in the art; of course, it is possible in a coupler of the present type to employ a fluid dielectric such asair.

As noted above, it is important that both ground planes of the coupler be moved equal and opposite amounts when varying the coupling between circuits. For many applications, it is desirable to provide a single means of moving both ground planes, and various different arrangements for this purpose are possible. In FIG. 4 there is schematically illustrated one mechanical arrangement for moving of the ground planes. Referring to this figure, it will be seen that a variable coupler 31 is illustrated as having metal top and bottom walls and side walls 33. Central conductors 34 and 36 of separate circuits are schematically illustrated in rotated position as being in close proximity with each other for parallel circuit coupling. Movable wall portions, or plugs, 37 and 38 are disposed in the top and bottom walls 32 at the coupling area, and these plugs are slidably disposed in the walls for movement toward and away from the central conductors 34 and 36.

Movement of the plugs 37 and 38 is accomplished by a micrometer arrangement 39, including a shaft 41 mounted for rotation in a block 42 secured to a side wall of the coupler. The shaft 41 is restrained from axial movement and has a threaded portion 43 at one end thereof and an oppositely threaded portion 44 at the other end thereof. A travelling nut 46 is disposed on the upper threaded portion 43 of the shaft with an arm 47 extended therefrom into engagement with the upper plug 37 in the coupler. A travelling nut 48 is similarly threaded on the lower threaded portion 44 of the shaft with an arm 49 extended therefrom into engagement with the lower plug .38 of the coupler. A wheel, or the like, 51 is mounted atop the shaft 41 for rotation of same, and it will be seen that as the wheel is turned, the nuts 46 and 48 move in opposite directions axially of the shaft to slide the plugs 37 and 38 toward each other at the same rate or away from each other at the same rate. Thus with the plugs of the coupler originally located equidistant from the central conductors 34 and 36- of the coupler, it is possible for an operator by merely turning the knob, or wheel, 51 slowly in either direction to move the plugs representing the ground planes of the coupler closer to the central conductor or further away from same to vary the coupling between circuits. It is not intended by this illustration and brief description to indicate that this type of mechanical motion of the ground planes is the only way of accomplishing same, but, instead, the illustration and description are provided solely for the purpose of identifying one operable embodiment of ground-plane-movement means.

The invention as described above operates to vary coupling between circuits of coaxial lines or striplines without the necessity of employing sliding contacts or mechanical motions of central conductors thereof. It is, however, noted that movement of the ground planes does introduce a slight mismatch, because of the varying internal volume of the coupler. Of course, at the primary, or nominal, coupling value there is no mismatch, because the coupler is designed to operate at this value. Provision may be made, however, to eliminate the small reflection, or mismatch, that would otherwise occur at adjusted positions of the ground plane for different coupling values. One manner of accomplishing this is illustrated in FIG. 5, wherein ground planes 61 and 61 are adapted to be moved toward each other or away from each other to vary coupling between central conductors 63 and 64 of the coupler 66. As these ground planes 61 and 62 are moved toward each other, for example, the side walls 67 and 68 are moved away from each other to maintain a constant volume within the coupler. This, of course, means that the ground planes 61 and 62 not only move vertically in the illustration, but, also, move laterally to maintain engagement with the side walls. Similarly, as the ground planes 61 and 62 are moved away from each other, the side walls 67 and 68 are moved inwardly toward each other; again, to maintain a constant volume Within the coupler. Mechanical arrangements for providing the above-noted motions of the coupler walls are not illustrated, as they are within the skill of those knowledgeable in the art.

An alternative embodiment of the present invention adapted to maintenance of a constant volume within the variable R-F coupler of the present invention is illustrated in FIGS. 6 and 7. Referring now to FIG. 6, it will be seen that the coupler includes upper and lower ground planes 71 and 72 disposed equidistantly upon opposite sides of central conductors 73 and 74. The two circuits being couled are represented by the common ground planes and separate central conductors, and no attempt is made in FIG. 6 to illustrate all details of the coupling structure. The illustration of FIG. 6 is primarily directed to illustration of structure for maintaining a constant volume within the coupler, and such will be seen in FIG. 6 to include side walls 76 and 77, formed of a flexible, electrically conducting material such as spring steel having the interior surface thereof silverplated. These side walls 76 and 77 are initially bowed outwardly, as illustrated in FIG. 6, and are firmly secured to the upper and lower ground planes 71 and 72, as by exterior welding 78. Thus, the outwardly bowed, flexible side walls 76 and 77 of the coupler are affixed to the upper and lower ground planes 71 and 72 to close the interior of the coupler.

In common with other embodiments of the present invention, the top and bottom walls, or ground planes, 71 and 72 of the coupler of FIG. 6 are movable toward each other and away from each other. In FIG. 7 there is illustrated the ground planes in 71 and 72 in displaced position from that of FIG. 6. In FIG. 7, the ground planes are shown to be located much closer to the central conductors 73 and 74, although yet equally spaced on opposite sides of the central conductors. As these top and bottom walls 71 and 72 are moved together, the flexible side walls 76 and 77 are additionally bowed outward to the position illustrated in FIG. 7. It will be seen that inasmuch as the side walls are firmly affixed to the top and bottom walls no change in internal volume of the coupler has occurred in moving the top and bottom walls together from the position of FIG. 7. Yet the degree of coupling between the two circuits has been changed by this movement. Consequently, variable coupling is provided by this embodiment, and yet problems of reflection or mismatch at varied coupling values are precluded. Again, it is noted that no attempt is made in the illustration to set forth all mechanical details of the coupler, for it is believed that the numerous possible variations will be apparent to those skilled in the art. It is particularly noted that in the embodiment of the present invention illustrated in FIGS. 6 and 7 there are no friction contacts across relatively highdensity current lines; consequently, no degrading of reliability is encountered.

There has been described above an improved variable R-F quarter-ware coupler for TEM mode circuits. It is to be appreciated that radio-frequency quarter-wave, TEM mode couplers are known in the art; furthermore that such prior-art devices have provided for variable coupling through the provision of variable spacing between central conductors of the separate circuits coupled. The present invention provides a material improvement in this type of device by provision for varying the spacing of the ground planes of the coupler. It is necessary that the groundplane spacing be changed equal and in opposite amounts for different coupling values. For applications wherein a slight mismatch, or reflection, can be tolerated for other than normal coupling values, the simple structure of FIG. 2 is highly advantageous. Under those circumstances wherein it is desired to eliminate even this slight mis-.

match, the embodiments of FIGS. 5 and 6 are highly advantageous.

The present invention is applicable for use as a matched attenuator or matched variable power divider for tight, or high, coupling values. For looser coupling values, the device of the present invention is particularly suited as a matched variable coupler for variable crystal sampling use or as a matched attenuator.

Although the present invention has been described with respect to particular preferred embodiments thereof, it is not intended to limit the invention to the actual details of illustration or terms of description.

That which is claimed is:

1. An improved variable TEM mode coupler comprising adjacent central conductors of separate circuits, means defining a pair of spaced ground planes common to both circuits to be coupled, said ground planes each including plates disposed equidistantly on opposite sides of said central conductors, and means controllably moving said plates equal distances toward said central conductors and away from said central conductors for varying coupling between said circuits.

2. The variable coupler of claim 1 further defined by said plates comprising flat-threaded screws on opposite sides of the coupling area for movement relative to said central conductors.

3. The variable coupler of claim 1 further defined by said means moving said plates. comprising a single threaded element having arms engaging said plates for sliding said plates equally toward and away from each other and said central conductors.

4. The variable coupler of claim 1 further defined by spaced side walls extending between said ground planes on opposite sides of said central conductors, and said side walls being movable to move equally inward toward said central conductors as said ground planes are moved outward and outwardly of said central conductors as said ground planes are moved toward each other to maintain a substantially constant coupling volume about said central conductors.

5. The variable coupler of claim 4 further defined by said side walls being comprised of flexible electrically conducting material bowed outwardly of said central conconductors between said ground planes and affixed to the means defining the ground planes, whereby movement of said plates flexes said side walls to maintain a substantially constant coupling volume about said central conductors.

6. An improved radio-frequency, quarter-wave, TEM mode coupler for at least two coaxial line circuits comprising a pair of spaced parallel ground planes common to both circuits, a central conductor from each circuit to be coupled and disposed in close proximity for a quarterwave length of center operating frequency of the circuits midway between said pair of ground planes in a coupling volume, electrically conducting side walls engaging said ground planes on opposite sides of said conductors, means controllably varying the ground plane spacing from the central conductors at said coupling volume in equal amounts to maintain the central conductors equidistant from the ground planes at all times and means changing the spacing of said side walls from said conductors in equal amounts and in an opposite sense to the variation in ground lane spacing from the conductors to thus vary the coupling between said circuits while maintaining the impedance of the coupler constant.

References Cited UNITED STATES PATENTS 3,407,366 10/1968 Dworkin 333-33X OTHER REFERENCES Singletary, Jr.: Fringing Capacitance in Strip-Line Coupler Design, IEEE Trans. on MTT, August 1966, p. 398.

PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 

